1. Field of the Invention
This invention relates generally to analog-to-digital (A/D) conversion systems and, more particularly, to a dual-slope A/D converter employing masked hysteresis.
2. Description of the Prior Art
Analog-to-digital (A/D) converters have been widely used in the electronics field for converting analog voltages to digital representations suitable for use with random logic circuits and, more recently, with microprocessors. Furthermore, integrated circuit technology is finding many more applications in the field of engine control, particularly with regard to controlling automobile engines. Whereas digital circuitry is usually characterized by its "ONE/ZERO" or "ON/OFF" nature, most measurements in the real world are inherently analog; e.g., temperature, pressure, speed, voltage, etc. Thus, A/D conversion is essential to permit digital processing of the analog signals.
When an analog signal crosses certain transition points, the digital output signal provided by the A/D converter is caused to change state. When the signal is in the vicinity of one of these transition voltages, noise may cause the voltage of the analog input signal to vary above and below the transition voltage causing the digital output to rapidly switch between the two states. This is especially true in an automotive environment where large voltage transients having magnitudes which vary from 100 to 200 volts are not uncommon.
One technique for reducing unwanted switching of the digital output due to noise is to add hysteresis to the switching circuitry. The introduction of hysteresis in switching circuits for reducing the effect of noise signals and improving noise margin is well known in the art. For example, a regenerative comparator or Schmidt trigger is shown in Strauss, Wave Generation and Shaping, 1970 McGraw-Hill Book Company, pp. 445-447. Furthermore, the addition of hysteresis in a multiplexed A/D system is described in copending application Ser. No. 863,760 filed Dec. 23, 1977 and assigned to the assignee of the present invention. A disadvantage to this approach, however, resides in the fact that the use of hysteresis with the threshold comparator of a dual-slope A/D converter produce an offset error in the conversion.